The trend of development of MRI technology has been to use magnets of increasing strengths. However, current high-field (static field B0 strength approximately 1.5 T and higher) MRI technology is hampered by non-uniform B, amplitude distribution offered by conventional coil techniques. At high B0 field strengths the size of the human brain or other body parts is no longer small in comparison to the wavelength of the RF inside tissues. For human brain imaging above 3 T, the center of the brain shows brighter image intensity compared with the periphery, due to wavelength effects. The Maxwell equations dictate that a single RF component (Bx or By) cannot be uniform over a three-dimensional (3D) volume under these conditions. It is possible to manipulate the RF field distribution to make it more uniform in one plane but the field homogeneity along the perpendicular direction will decrease. Consequently, it has been very difficult to obtain a highly uniform 180° pulse through a large 3D sample volume. As used in the entire document, “field amplitude”, “B1 field amplitude” and “RF field amplitude” all mean the amplitude of the transverse component of the B1 field unless specifically indicated otherwise, because the longitudinal component does not contribute to spin excitation.
Achieving uniform RF excitation in high field MRI scanners is not a trivial task. In the case of brain scans, because of the large relative dielectric constant of water in the tissue, images acquired with conventional volume coils show higher signal intensity at the center of the head. Although a totally homogeneous B1 field is unattainable over a 3D volume, allowing phase variation in space can lead to improvement in the B1 amplitude distribution. Modification of birdcage coils showed improvement in the homogeneity of RF excitation. Due to the short wavelength of the RF field inside the tissue, the field distribution is strongly affected by the shape of the object, and conventional hardware configurations do not provide optimal results at very high field strengths. Many approaches have been proposed to improve RF field homogeneity, including a multiple-port transverse electromagnetic (TEM) resonator driven with variable phase and amplitude at each port (Magn Reson Imaging 2001: 19: 1339-1347) and the use of an array coil to obtain uniform Bx and By in one plane (J Magn Reson Imaging 2000: 12: 46-67).
The RF magnetic field for spin excitation can be described by a steady state field B(r,t)=B(r)·e−iωt. Achieving uniform RF excitation in high field MRI scanners is desirable, but not a trivial task. Because of the large relative dielectric constant of water in the tissue, images acquired with conventional volume coils show higher signal intensity at the center of the head. (J Magn Reson 2004: 167: 12-24). Although a totally homogeneous B1 field is unattainable (J Magn Reson Imaging 2000: 12: 46-67), allowing phase variation in space can lead to improvement in the B1 amplitude distribution. Modification of birdcage coils showed improvement in the homogeneity of RF excitation. (Magn Reson Med 1992: 23: 287-301; Magn Reson Med 1998: 40: 49-54). Due to the short wavelength of the RF field inside the tissue, the field distribution is strongly affected by the shape of the object, and conventional hardware configurations are unable to provide optimal results at very high field strengths. Many approaches have been proposed to overcome this difficulty, including a multiple-port transverse electromagnetic (TEM) resonator driven with variable phase and amplitude at each port. (Magn Reson Imaging 2001: 19: 1339-1347). The use of an array coil to obtain uniform Bx and By field in one plane (J Magn Reson Imaging 2000: 12: 46-67) has also been proposed.